Bias Adjustment Factors

Diffusion tubes are useful low-cost method for indicative monitoring of ambient nitrogen dioxide (NO2) concentrations. However, diffusion tubes are affected by several sources of interference which can cause substantial under or overestimation (often referred to as "bias") compared to the chemiluminescent analyser (defined within Europe as the reference method).

Clearly, any such "bias" is a problem in any situation where diffusion tube results are to be compared with air quality objectives. As a result, local authorities using NO2 diffusion tubes in their Review and Assessment are required to quantify the "bias" of their diffusion tube measurements and apply an appropriate bias adjustment factor to the annual mean if required.

Local Authorities can either:

Carry out their own co-location study (in which the accuracy of the diffusion tubes is quantified by exposure alongside an automatic chemiluminescence analyser), and use the results to calculate a bias adjustment factor. For more information please seeLocally-derived bias adjustment factors.

Use a combined bias adjustment factor, based on the result of many co-location studies (using the same laboratory and tube preparation method). For further information on combined bias adjustment factors, please seeNational bias adjustment factors.

But which of these is best - the result of a local co-location study, or the average of as many co-location studies (for the laboratory and tube preparation method) as possible? The answer to this question depends on a number of factors. Please refer to Box 7.11 of the LAQM.TG16 technical guidance for more details.

Information on Precision and Accuracy is provided here. You can also download a summary of precision results for the individual laboratories for 2008-2017 from this page.